The development of instruments to quantitate health-related quality of life (HRQOL) includes taking the appropriate and, in fact, essential steps to ensure that the designated tool measures what it purports to measure and does it accurately and reliably.

Such steps are not only critical in the development of appropriate tools, but, when clearly presented, also serve to prove to the reader (and reviewers) that the instrument is a legitimate one.

Key to the development of any HRQOL instrument are three fundamental concepts : validity, reliability, and responsiveness.

Validity proves that the instrument measures what it claims to measure.

Reliability proves that the results are reproducible in a stable setting.

Responsiveness proves that the instrument is sensitive enough to record important differences or changes.

Each of these principles is discussed, including some examples of the application of these principles in previously published HRQOL studies.

The studies referenced are excellent examples of the methods used in the creation of a HRQOL scale, and the reader is strongly encouraged to review these in their entirety to fully grasp the concepts.